Development of flat- and self-light-emitting-type display apparatuses in which organic EL devices are used as light-emitting devices has been actively conducted in recent years. Organic EL devices are devices utilizing a phenomenon where applying an electric field to an organic thin film causes light emission. Since organic EL devices are driven at an applied voltage of 10 V or less, low power consumption is required. In addition, since organic EL devices are self-light-emitting devices that emit light by themselves, illuminating members are not necessary and thus weight-lightening and thinning can be easily achieved. Furthermore, since the response speed of organic EL devices is very high, such as about several microseconds, residual images at the time when moving images are displayed are not generated.
Among flat- and self-light-emitting-type display apparatuses in which organic EL devices are used in pixels, in particular, development of active-matrix-type display apparatuses in which thin-film transistors are integrated and formed as driving devices in each of pixels has been actively conducted. Active-matrix-type flat and self-light-emitting display apparatuses are described, for example, in patent documents 1 to 5 listed below.
[Patent Document 1] Japanese Unexamined Patent Application Publication No. 2003-255856
[Patent Document 2] Japanese Unexamined Patent Application Publication No. 2003-271095
[Patent Document 3] Japanese Unexamined Patent Application Publication No. 2004-133240
[Patent Document 4] Japanese Unexamined Patent Application Publication No. 2004-029791
[Patent Document 5] Japanese Unexamined Patent Application Publication No. 2004-093682
FIG. 24 is a schematic circuit diagram showing an example of a known active-matrix-type display apparatus. The display apparatus is constituted by a pixel array unit 1 and peripheral driving units. The driving units include a horizontal selector 3 and a write scanner 4. The pixel array unit 1 includes signal lines SL in columns and scanning lines WS in rows. Pixels 2 are arranged in portions where respective signal lines SL and respective scanning lines WS intersect with each other. For the sake of easier understanding, only one pixel 2 is illustrated in the figure. The write scanner 4 includes shift registers. The write scanner 4 operates in accordance with clock signals ck that are supplied from the outside and sequentially transfers start pulses sp that are also supplied from the outside, so that the write scanner 4 sequentially outputs control signals to the scanning lines WS. The horizontal selector 3 supplies video signals to the signal lines SL in accordance with line-sequential scanning by the write scanner 4.
The pixels 2 are each constituted by a sampling transistor T1, a driving transistor T2, a holding capacitor C1, and a light-emitting device EL. The driving transistor T2 is of a P-channel type. The source of the driving transistor T2 is connected to a power supply line, and the drain of the driving transistor T2 is connected to the light-emitting device EL. The gate of the driving transistor T2 is connected to a signal line SL with the sampling transistor T1 therebetween. The sampling transistor T1 is brought into conduction in accordance with a control signal supplied from the write scanner 4, and samples a video signal supplied from the signal line SL to write the video signal to the holding capacitor C1. The driving transistor T2 receives as a gate voltage Vgs, at the gate thereof, the video signal written to the holding capacitor C1, and causes a drain current Ids to flow to the light-emitting device EL. Accordingly, the light-emitting device EL emits light at a brightness corresponding to the video signal. The gate voltage Vgs represents the potential of the gate, which is based on the source.
The driving transistor T2 operates in a saturation region. The relationship between the gate voltage Vgs and the drain current Ids is represented by the following characteristic equation:Ids=(½)μ(W/L)Cox(Vgs−Vth)2,
where μ represents the mobility of the driving transistor, W represents the channel width of the driving transistor, L represents the channel length of the driving transistor, Cox represents the gate insulation capacitance of the driving transistor, and Vth represents the threshold voltage of the driving transistor. As is clear from the characteristic equation, in a case where the driving transistor T2 operates in the saturation region, the driving transistor T2 functions as a constant-current source that supplies the drain current Ids in accordance with the gate voltage Vgs.
FIG. 25 is a graph showing the voltage/current characteristics of the light-emitting device EL. The abscissa represents an anode voltage V, and the ordinate represents the driving current Ids. In addition, the anode voltage of the light-emitting device EL is the drain voltage of the driving transistor T2. The current/voltage characteristics of the light-emitting device EL change with time, and the characteristic curve tends to become flatter as time passes. Thus, even if the driving current Ids is constant, the anode voltage (drain voltage) V changes. In this respect, in the pixel circuit 2 shown in FIG. 24, since the driving transistor T2 operates in the saturation region and the driving current Ids corresponding to the gate voltage Vgs can thus be caused to flow irrespective of a variation in the drain voltage, a constant light-emission brightness can be maintained irrespective of a time-lapse change in the characteristics of the light-emitting device EL.
FIG. 26 is a circuit diagram showing another example of a known pixel circuit. A difference from the pixel circuit shown above in FIG. 24 is that the driving transistor T2 is changed from being of the P-channel type to being of an N-channel type. In terms of the manufacturing process of a circuit, it is often advantageous to use N-channel-type transistors for all the transistors forming a pixel.